Redesign access to advertising data for a single device.

[scheib]

Current spec for access to the advertising data of a single device appears as:

interface BluetoothDevice {
  readonly attribute BluetoothAdvertisingData adData;
  ...
}
interface BluetoothAdvertisingData {
  readonly attribute unsigned short? appearance;
  readonly attribute byte? txPower;
  readonly attribute byte? rssi;
  readonly attribute BluetoothManufacturerDataMap manufacturerData;
  readonly attribute BluetoothServiceDataMap serviceData;
};

However, as discussed on https://code.google.com/p/chromium/issues/detail?id=568137 (ortuno & jyasskin) this is limited in many ways:

• You have to be actively scanning to get the TxPower and the RSSI
• Many devices don't advertise if they are connected to.
• comment 6 details on RSII updates from Adv packets and connected device polling.

We could safely let web sites "scan" for devices they already have access to, which would let them opt into the increased power use and get advertising data updates. I think we'd change the .adData field into a "watch" function and event. Thoughts?

This issue is distinct from issue Let websites scan for nearby advertisements which would scan for all advertisements. Though, perhaps if #191 is solved this issue may be omitted as a subset of what could be done scanning for all AD packets.